Further recovery of crude oil remaining in porous subterranean formations after primary drilling and recovery operations are no longer effective is often accomplished by flooding the oil-bearing formation to force the trapped oil out of the sand or rock matrix toward a secondary recovery well. Water flooding is often employed as a secondary recovery technique, but because water readily flows through porous oil-bearing formations, significant amounts of crude oil will not be displaced. This has lead to the development of injection drive fluids, which have sufficient flowability to permeate the oil-bearing formation but have proper viscosity to displace trapped oil toward a recovery well. Several patents, including U.S. Pat. Nos. 3,476,186 (Sarem), 3,825,067 (Vestal), 3,868,997 (Pogers), 4,113,688 (Pearson), British Pat. No. 2,100,611 and British Pat. No. 2,101,179 (Marathon Oil), all of which are incorporated herein by reference, describe the preparation and use of injection drive fluids comprising water and a small amount of a water-soluble polymer, such as a polyacrylamide. Ideally, the water-soluble polymer of such an injection drive fluid will be of sufficient molecular weight to create the proper viscosity to displace the oil in an oil-bearing formation, but the polymer cannot be so high in molecular weight as to stop or clog the porous formation, and the drive fluid must be sufficiently flowable to permeate the porous formation.
Polyacrylamide gel is formed by polymerization of acrylamide monomer in aqueous solution. The polymer gel is a semi rigid rubbery material that requires further handling to convert it into the dilute aqueous polymer drive fluid. The gel must be ground or chopped into small pieces, dispersed in water and time given to allow the polymer to dissolve completely.
Prior methods have been unsatisfactory in that excessive shear or mixing degrades the polymer, lowering the polymer molecular weight and giving a solution which will be less effective for displacing trapped oil. In addition, even where excessive shear degradation is avoided, the multiple dilution steps and/or long holding times required to obtain uniform solutions without degrading the polymer makes these methods cumbersome, expensive and slow, and if the solutions are prepared at the drilling site, large storage capacities must be dedicated to solution preparation.
A process has now been discovered for converting polyacrylamide gel into aqueous polyacrylamide injection drive fluid which allows continuous on-site production at low cost of polyacrylamide drive fluids tailored to the particular subterranean formation into which they will be pumped.
The process involves making a slurry of small gel particles in water and holding the solution in a single hold tank (batch process) or a series of 3 hold tanks (continuous process) to form a homogeneous polymer solution concentrate of moderate polymer solids (0.5%-2.0%) called "polymer solution concentrate". From this "polymer solution concentrate" the final dilute (0.1-0.5% polymer solids) fluid drive solution is easily obtained by dilution with water using standard static mixer dilution procedures. This solution can be used immediately without the need of further holding or homogenizing steps. Besides reducing production time and eliminating the need for some storage equipment, the process of the present invention avoids polymer degradation, is readily adapted to the drilling site and can operate with available water, even sea water.